Electoviscous liquids (EVLS) are dispersions of finely divided hydophilic solids in hydophobic, electrically non-conductive oils of which the viscosity may be increased very quickly and reversibly from the liquid to the plastic or solid state under the effect of a sufficiently strong electrical field. Their viscosity responds both to electrical d.c. fields and to a.c. fields. The current flowing through the EVL should be extremely low. Accordingly, EVLS may be used for any applications in which it is desired to control the transmission of powerful forces by low electric energy consumption, for example in clutches, hydraulic valves, shock absorbers, vibrators or systems for positioning and holding workpieces in position.
In addition to the requirements which an EVL generally has to satisfy, such as a good electroviscous effect, high temperature stability and chemical stability, the abrasiveness and anti-settling behavior of the disperse phase play an important part in practical application. Ideally, the disperse phase should not sediment, but at all events should be readily redispersible and should not cause any abrasion under extreme mechanical wear.
According to some prior art teachings, the disperse phase consists of organic solids, such as for example saccharide (DE 25 30 694), starch (EP 284 268 A2, U.S. Pat. No. 3,970,573), polymers (EP 150 994 A1, DE 33 10 959 A1, GB 1,570,234, U.S. Pat. No. 4,129,513), ion exchanger resins (UP 92278/1975, JP 32221/1985, U.S. 3,047,507), or silicone resins DE 39 12 888 A1).
Inorganic materials have also been used, including for example Li hydrazine sulfate (U.S. Pat. No. 4,772,407 A), zeolites (EP 265 252 A2), silica gel (DE 35 17 281 A1, DE 34 27 499 A1), aluminum silicates (DE 35 36 934 A1), TiO.sub.2 (SU 715 596), BaTiO.sub.3 (JP 53/17585) or metal powders, such as aluminum (UP 016093, UP 01172496). These solids were dispersed, in some cases with the aid of dispersants, in non-conductive, partly substituted liquids, such as for example hydrocarbons, aromatic hydrocarbons and silicone oil.
The disperse phase of these EVLS sediments in some cases very quickly, and is abrasive on account of the hardness of the particles dispersed therein.
Abrasion can be influenced to a large extent by the choice of the disperse phase. Polymeric substances are preferred to inorganic powders as the disperse phase.
Attempts have been made to solve the problem of sedimentation by using liquid phases of high specific gravity. Through the increase of the specific gravity of the liquid, for example by using fluorinated, chlorinated or brominated hydrocarbons, the difference in density between the liquid phase and the disperse phase decreases as, hence, does the sedimentation of the said particles. For example, to disperse the solids lithium polyacrylate, silica gel and salts of a crosslinked polymethacrylic acid, U.S. Pat. No. 4,502,973 uses halogenated diphenyl methane, EP-PS 284 268 A2 uses polychlorotrifluoroethylene and DE-PS 33 10 959 uses brominated diphenyl methane as the liquid phase. However, substituted liquids are generally not environment-friendly.
DE-OS 30 12 888 A1 describes a non-sedimenting EVL containing a fine powder of a silicone resin dispersed in an electrically insulating oil. However, these liquids have a relatively poor electroviscous effect.